1. Technical Field
The present invention relates to electrocatalysts, particularly Pd and Pt-based electrocatalysts on a WO3-ordered mesoporous carbon support, their use in direct formic acid fuel cells for portable electronic device applications and a process of electro-catalytic oxidation of formic acid.
2. Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
In recent years direct formic acid fuel cells (DFAFCs) have received growing interest as a compact generator, for electronic devices and transportation means. In a DFAFC, formic acid oxidation (FAO) takes place at the anode side while reduction occurs at the cathode side (Cynthia A R, Akshay B, Peter G P. Recent Advances in Electrocatalysis of Formic Acid Oxidation. Springer London, Lecture Notes in Energy 9(2013) 69-87—incorporated herein by reference in its entirety). Generally, the DFAFC offers the following major advantages: (1) safe and easy to handle, non-toxic, (2) can create high theoretical open circuit potential of 1.48 V which is larger than hydrogen (1.23 V) and methanol (1.21 V), and (3) low cross over through membrane than methanol and ethanol. Although formic acid has lower energy density (2104 Wh/L) compared to methanol (4900 Wh/L), the low cross over through the membranes allows the DFAFCs to operate at high formic acid concentrations (5-12 M) compared to methanol concentration (1-2 M), ensuing overall higher energy outputs (Cynthia A R, Akshay B, Peter G P. Recent Advances in Electrocatalysis of Formic Acid Oxidation. Springer London, Lecture Notes in Energy 9(2013) 69-87; Olumide W, Zhiyong Z, Changhai L, Wenzhen L., Electrochim. Acta 2011; 55(13): 4217-4221 W L Qu, Z B Wang, X L Sui, D M Gu, G P Yin, Fuel Cells 2013; 13 (2): 149-157; Zhiming C, Cheng G, Chun X G and Chang M L., J. Mater. Chem. A, 1(2013) 1179-1184—each incorporated herein by reference in its entirety).
In addition to the above technical advantages, mass scale applications of DFAFCs can also create opportunities of utilizing CO2 (from fossil fuel combustion) as a source of formic acid production via electrochemical conversion of carbon dioxide (Charles D, Paul L. R. John B. K and John N., Electrochem. Soc. (2008); 155 (1): 42-49; Hui Li and Oloman, C. Continuous co-current electrochemical reduction of carbon dioxide. WO2007041872 B1, 2007—each incorporated herein by reference in its entirety). This integrated approach not only offers DFAFCs as way of efficient energy generator but also contributes to the global efforts on the CO2 utilization/sequestration, addressing the greenhouse gas effects (Charles D, Paul L. R, John B. K and John N., Electrochem. Soc. (2008); 155 (1): 42-49; Hui Li and Oloman, C. Continuous co-current electrochemical reduction of carbon dioxide. WO2007041872 B1, 2007—each incorporated herein by reference in its entirety). The process of converting CO2 to formic acid will be only economically viable if the energy demand for the electrochemical conversion of CO2 to formic acid is supplemented from a renewable source such as solar energy. With the global efforts and recent advancements of the solar technologies, the outlook remains positive for this integrated approach.
In order to materialize the aforementioned advantages of DFAFCs, research and development efforts are underway. Despite some advancement, the present DFAFC systems suffer from some practical issues which need to be addressed in order to exploit their full benefits. The foremost drawback of the present DFAFCs is the use of expensive and scarce noble metal-based electrocatalysts to accelerate the slow kinetics of the anodic electro-oxidation of formic acid (Cynthia A R, Akshay B, Peter G P. Recent Advances in Electrocatalysis of Formic Acid Oxidation. Springer London, Lecture Notes in Energy 9(2013) 69-87; Olumide W, Zhiyong Z, Changhai L, Wenzhen L., Electrochim. Acta 2011; 55(13): 4217-4221; Yang L Jun, Su Neng, Ting and Shi Jun, Sci China Chem. 55(2012) 391-397; Feng, L G; Yang, J; Hu, Y; Zhu, J B; Liu, C P Xing, W., Int. J. Hydrogen Energ. 37(2012) 4812-4818; Yuan H Qin, Yue-J, Hou-H Y, Xin S Z, Xing G Z, Li N, Wei K Y., J. Power Sources 196 (10)(2011) 4609-4612—each incorporated herein by reference in its entirety). In addition to their high costs, the noble metal-based catalysts also suffer from severe poisoning due to the strong adsorption of the carbon monoxide (Haan J L, Masel R. I., Electrochim. Acta 54(2009) 4073-4078—incorporated herein by reference in its entirety) and chemical instability in acidic environment. Among the noble metals, Pt and Pd are extensively studied as active components of the anode electrocatalysts. Although, Pd-based electrocatalysts showed higher catalytic activity for FAO reactions than Pt, it still lacks stability for the long period of operations (Yu Zhu, Yongyin K, Zhiqing Z, Qun Z, Junwei Z, Baojia X and Hui Y., Electrochem. Commun. 10(2008) 802-805; Xiao M Wang, Yong Y X., Electrochim. Acta 54(2009) 7525-7530—each incorporated herein by reference in its entirety). These difficulties warrant further research to develop highly durable and efficient Pd-based electrocatalysts for DFAFCs. In the open literature, various metals have been explored as promoter to enhance the catalytic activity and stability of Pd catalysts. The use of transition metals also helps reducing the use of noble metals in the catalyst formulation while maintaining or even improving the catalytic activity. The most common studied bimetallic catalysts include PdCo, PdNi, PdAu, PdPt, PtBi, PdSn and PdFe (Lu Zhang, Ling W, Yanrong Ma, Yu C, Yiming Z, Yawen T, Tianhong L., Appl. Catal. B, Environ. 138-139(2013) 229-235; Rongfang Wang, Hui W, Xingli W, Shijun L, Vladimir L and Shan J., Int. J. Hydrogen Energ. 38(2013) 13125-13131; Maja D., Obradovic, Sne, Gojkovi, Electrochim. Acta 88(2013) 384-389; Zhao, Zhua, Liuc and Wei Xing, Appl. Catal. B: Environ. 129(2013) 146-152; Zhang, Chun He, Jiang, Rao and Shi-Gang Sun, Electrochem. Commun. 25(2012) 105-108; DandanTu, Bing, Wang, Deng and Ying Gao, Appl. Catal. B: Environ. 103(2011) 163-168; Yanxian Jin, Chun'an M, Meiqin S, Youqun C, Yinghua X, Tao H, Qian H and Yiwai M., Int. J. Electrochem. Sci. 7(2012) 3399-3408—each incorporated herein by reference in its entirety).
Conventionally, the active metals/promoters are dispersed on a suitable support material to achieve highest possible catalytic activity using a minimum amount of metal. The supports also provide the required strength to the electrocatalyst in acidic environment of the fuel cells (Ermete Antolini., Appl. Catal. B: Environ. 88(2009) 1-24—incorporated herein by reference in its entirety). Like the conventional supported catalysts, high surface area, large pore volume and superior electrical conductivity of the support is highly desirable. The high surface area of the support allows better dispersion and less agglomeration of the nano-sized active metal particles, resulting in optimum catalytic performance. Among the studied support materials, large surface area carbon such as Vulcan XC72 carbon black is possibly the most widely used in electrocatalysts. With some advantages there are drawbacks of Vulcan XC72 carbon black supported electrocatalysts. Among those, the most important is non-contribution of some of the loaded expensive noble metals particles which are trapped in the deep cracks of the phase boundaries and micropores of the carbon black support (Yuyan Shao, Geping Y, Jiajun W, Yunzhi G and Pengfei S., J. Power Sources (2006); 161 (1): 47-53—incorporated herein by reference in its entirety). Carbon black also suffers from serious corrosion problems in the fuel cell oxidation operation (Sudong Yang, Xiaogang Z, Hongyu M, Xiangguo Y., J. Power Sources 175(2008) 26-32; Bruce R. R J. Frank R. M and Elton J. C., J. Electrochem. Soc. 142(1995) 1073-1084—each incorporated herein by reference in its entirety). In order to avoid these problems there are many other carbon materials have been investigated as electrocatalyst support, including carbon nanotubes (CNTs) (Chun'an M., Yanxian J., Meiqin S., Youqun Ch., Yinghua X., Wenping J., Qiaohua Y., Jiabin C., Dongkai C., Shuomiao C., Journal of the Electrochemical Society 161(2014): F246-F251; Olumide W, Zhiyong Z, Changhai L, Wenzhen L., Electrochim. Acta 2011; 55(13): 4217-4221; Zhiming C, Cheng G, Chun X G and Chang M L., J. Mater. Chem. A, 1(2013) 1179-1184; Yanxian Jin, Chun'an M, Meiqin S, Youqun C, Yinghua X, Tao H, Qian H and Yiwai M., Int. J. Electrochem. Sci. 7(2012) 3399-3408; Yuyan Shao, Geping Y, Jiajun W, Yunzhi G and Pengfei S., J. Power Sources (2006); 161 (1): 47-53; Sudong Yang, Xiaogang Z, Hongyu M, Xiangguo Y., J. Power Sources 175(2008) 26-32—each incorporated herein by reference in its entirety), nanofibers (CNFs) (Yuan H Qin, Yue-J, Hou-H Y, Xin S Z, Xing G Z, Li N, Wei K Y., J. Power Sources 196 (10)(2011) 4609-4612—incorporated herein by reference in its entirety) ordered mesoporous carbon (OMCs) (J. Zeng, C. Francia, C. Gerbaldi, V. Baglio, S. Specchia, A. S. Aricò, P. Spinelli., Electrochim. Acta 94(2013) 80-91; Sang H Joo, Seong J C, Ilwhan O, Juhyoun K, Zheng L, Osamu T & Ryong R., Nature 412(2001) 169-172; Sang Hoon Joo, Chanho P, Dae J Y, Seol-Ah L, Hyung I L, Ji M K, Hyuk C, Doyoung S., Electrochim. Acta 52(2006) 1618-1626; Zhi-Peng Sun, Xiao G Z, Hao Tong, Yan Y L, Hu L L., J. Colloid and Interf. Sci. 337(2009) 614-618; Juqin Zeng, Carlotta F, Mihaela A. D, Alessandro H. A. M V, Vijaykumar S. I, Stefania S, and Paolo S., Ind. Engg. Chem. Res. 51(2012) 7500-7509; Chuntao L, Meng C, Chunyu D, Jing Z, Geping Y, Pengfei S and Yongrong Sun., Int. J. Electrochem. Sci. 7(2012) 10592-10606—each incorporated herein by reference in its entirety), graphene (Seger B and Kamat P V., J. Phys. Chem. C (2009); 113(19): 7990-95—incorporated herein by reference in its entirety), metal carbides (Dong J H and Jae S L., Energies 2009; 2(4): 873-899—incorporated herein by reference in its entirety) among others.
Amongst the above support materials, ordered mesoporous carbons (OMCs) have found a wide range of potential applications due to their uniform pore structure, large pore volumes, high surface areas, superior electrical conductivity and good chemical stability (Zhang, Chun He, Jiang, Rao and Shi-Gang Sun, Electrochem. Commun. 25(2012) 105-108; J. Zeng, C. Francia, C. Gerbaldi, V. Baglio, S. Specchia, A. S. Aricò, P. Spinelli., Electrochim. Acta 94(2013) 80-91; Sang H Joo, Seong J C, Ilwhan O, Juhyoun K, Zheng L, Osamu T & Ryong R., Nature 412(2001) 169-172; Sang Hoon Joo, Chanho P, Dae J Y, Seol-Ah L, Hyung I L, Ji M K, Hyuk C, Doyoung S., Electrochim. Acta 52(2006) 1618-1626; Zhi-Peng Sun, Xiao G Z, Hao Tong, Yan Y L, Hu L L., J. Colloid and Interf. Sci. 337(2009) 614-618; Juqin Zeng, Carlotta F, Mihaela A. D, Alessandro H. A. M V, Vijaykumar S. I, Stefania S, and Paolo S., Ind. Engg. Chem. Res. 51(2012) 7500-7509; Chuntao L, Meng C, Chunyu D, Jing Z, Geping Y, Pengfei S and Yongrong Sun., Int. J. Electrochem. Sci. 7(2012) 10592-10606—each incorporated herein by reference in its entirety). When a suitable noble metal was deposited on OMCs, the resultant electrocatalysts showed excellent performances on methanol oxidations in a methanol fuel cell (J. Zeng, C. Francia, C. Gerbaldi, V. Baglio, S. Specchia, A. S. Aricò, P. Spinelli., Electrochim. Acta 94(2013) 80-91; Sang H Joo, Seong J C, Ilwhan O, Juhyoun K, Zheng L, Osamu T & Ryong R., Nature 412(2001) 169-172—incorporated herein by reference in its entirety). It has also been used as a support in a Pt-based electrocatalyst for formic acid fuel cell (Chuntao L, Meng C, Chunyu D, Jing Z, Geping Y, Pengfei S and Yongrong Sun., Int. J. Electrochem. Sci. 7(2012) 10592-10606—incorporated herein by reference in its entirety).
The modification of the ordered mesoporous carbon support material is also found beneficial to improve the activity of supported catalysts. Partially filled d- or f-orbital of the transition metals allow them to switch between valences. Metal oxide-carbon composites have been extensively investigated as support material for methanol oxidation electrocatalysts (W L Qu, Z B Wang, X L Sui, D M Gu, G P Yin, Fuel Cells 2013; 13 (2): 149-157; Yang L Jun, Su Neng, Ting and Shi Jun, Sci China Chem. 55(2012) 391-397; J. Zeng, C. Francia, C. Gerbaldi, V. Baglio, S. Specchia, A. S. Aricò, P. Spinelli., Electrochim. Acta 94(2013) 80-91; Min K J, Jung Y W, Ki R L, Seong I W., Electrochem. Commun. 9(2007) 2163-2166; Gumaa El-Nagar, Ahmad M. Mohammad, El-Deab and El-Anadouli, Electrochim. Acta 94(2013) 62-71; Hao An, Cui, Zhou and Dejing Tao, Electrochim. Acta 92 (2013) 176-182—each incorporated herein by reference in its entirety). These studies showed that the addition of metal oxide improves both the activity and stability of the catalysts. There are other reports discussing the modification effects of NiO, WO3 and CeO2 on Pd/Pt-C for FAO (Yang L Jun, Su Neng, Ting and Shi Jun, Sci China Chem. 55(2012) 391-397; J. Zeng, C. Francia, C. Gerbaldi, V. Baglio, S. Specchia, A. S. Aricò, P. Spinelli., Electrochim. Acta 94(2013) 80-91; Gumaa El-Nagar, Ahmad M. Mohammad, El-Deab and El-Anadouli, Electrochim. Acta 94(2013) 62-71—each incorporated herein by reference in its entirety). In general, the addition of the transition metal oxides improves the overall performance of the carbon supported Pd-based electrocatalysts.
To achieve the highest attainable electrocatalytic activity of catalyst, OMC support with uniform pore structure, large mesopores volume and high specific surface area is modified with metal oxide (WO3) nanoparticles (Zhang, Chun He, Jiang, Rao and Shi-Gang Sun, Electrochem. Commun. 25(2012) 105-108; J. Zeng, C. Francia, C. Gerbaldi, V. Baglio, S. Specchia, A. S. Aricò, P. Spinelli., Electrochim. Acta 94(2013) 80-91; Sang H Joo, Seong J C, Ilwhan O, Juhyoun K, Zheng L, Osamu T and Ryong R., Nature 412(2001) 169-172; Sang Hoon Joo, Chanho P, Dae J Y, Seol-Ah L, Hyung I L, Ji M K, Hyuk C, Doyoung S., Electrochim. Acta 52(2006) 1618-1626; Zhi-Peng Sun, Xiao G Z, Hao Tong, Yan Y L, Hu L L., J. Colloid and Interf. Sci. 337(2009) 614-618; Juqin Zeng, Carlotta F, Mihaela A. D, Alessandro H. A. M V, Vijaykumar S. I, Stefania S, and Paolo S., Ind. Engg. Chem. Res. 51(2012) 7500-7509; Chuntao L, Meng C, Chunyu D, Jing Z, Geping Y, Pengfei S and Yongrong Sun., Int. J. Electrochem. Sci. 7(2012) 10592-10606—each incorporated herein by reference in its entirety). Previous studies showed that commonly used modifiers such as TiO2, WO3, CeO2, ZrO2, NiO, and Fe2O3 are found beneficial to improve the activity of supported catalysts (W L Qu, Z B Wang, X L Sui, D M Gu, G P Yin, Fuel Cells 2013; 13 (2): 149-157; Yang L Jun, Su Neng, Ting and Shi Jun, Sci China Chem. 55(2012) 391-397; J. Zeng, C. Francia, C. Gerbaldi, V. Baglio, S. Specchia, A. S. Aricò, P. Spinelli., Electrochim. Acta 94(2013) 80-91; Min K J, Jung Y W, Ki R L, Seong I W., Electrochem. Commun. 9(2007) 2163-2166; Gumaa El-Nagar, Ahmad M. Mohammad, El-Deab and El-Anadouli, Electrochim. Acta 94(2013) 62-71; Hao An, Cui, Zhou and Dejing Tao, Electrochim. Acta 92 (2013) 176-182—each incorporated herein by reference in its entirety). Particularly, tungsten trioxide (WO3) showed promising characteristics as a support modifier for formic acid (K. Y. Chen, P. K. Shen, A. C. C., Electrochem. Soc. 142 (1995) L54-L56; Z. H. Zhang, Y. J. Huang, J. J. Ge, C. P. Liu, T. H. Lu, W. Xing. Electrochem. Commun. 10 (2008) 1113-1116—each incorporated herein by reference in its entirety) and methanol fuel cell (S. Sharma, B. G. Pollet, J. Power Sources 208 (2012) 96; E. Antolini, E. R. Gonzalez, Appl. Catal. B: Environ. 96 (2010) 245; A. S. Aricò, V. Baglio, V. Antonucci, Direct methanol fuel cells: history, status and perspectives, in: H. Liu, J. Zhang (Eds.), Electrocatalysis of Direct Methanol Fuel Cells, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2009, p. 1 (Chapter 1); P. K. Shen, A. C. C. Tseung, Electrochem. Soc. 141 (1994) 3082—each incorporated herein by reference in its entirety) electro-oxidation. Zhang et al. reported that Pd nanoparticles deposited on hybrid WO3 support displayed high electro-oxidation activity for formic acid. Feng et al. (reported improved performance of formic acid oxidation using a WO3/C hybrid support for a Pd based electrocatalysts (L Feng, L Yan, Z Cui, C Liu and Wei Xing., J. Power Sources 196(5) (2011) 2469-2474—incorporated herein by reference in its entirety). In the above catalysts, WO3 acts both as a support modifier and promoter of the noble metals (Zhang, Y. J. Huang, J. J. Ge, C. P. Liu, T. H. Lu, W. Xing. Electrochem. Commun. 10 (2008) 1113-1116—incorporated herein by reference in its entirety). It is reported that WO3 facilitates the formation of hydrogen bronze (HxWO3), which enhances the rate of dehydrogenation during oxidation in acidic medium (B. S. Hobbs, A. C. C. Tseung., Nature 222 (1969) 556-558—incorporated herein by reference in its entirety). The oxophilic nature of WO3 helps removing the adsorbed CO intermediates from the Pt metal surface during the oxidation steps (A. C. C. Tseung, K. Y. Chen., Catalysis Today 38 (1997) 439-443—incorporated herein by reference in its entirety). The presence of WO3 can also create a barrier phase between the support and the active noble metals slowing down the catalyst deactivation as occurs due to active metal particle agglomeration. Thus tungsten oxide can play an important role in further improvement of the commercially available carbon black supported catalysts improving both the catalytic performance and CO tolerance for fuel cell anodic catalyst electrodes (Hao An, Cui, Zhou and Dejing Tao, Electrochim. Acta 92(2013) 176-182—incorporated herein by reference in its entirety). To the best knowledge of the applicants, there are only few reports available in the open literature on the use of WO3-OMC hybrid material as a catalyst support for the electro-oxidation.
In view of the foregoing, the need for improvements in WO3-modified OMC as support material for Pd/Pt-based electrocatalyst for formic acid oxidation in DFAFCs and the need for improvement to Pd/Pt-based electrocatalyst for formic acid oxidation can readily be appreciated.